It depends on the class! French, Spanish, and most Contemporary Topics classes do not require the purchase of a textbook and will use free online resources. For all other classes, textbooks typically cost between $40 - $110 and are used throughout a variety of levels. The six evening group program pages by language provide material links for the student to purchase if applicable.
Just so you know, you should not purchase textbooks until you have received confirmation from ICLS about two weeks before the course starts.